Frequency selective surfaces can be provided to selectively reduce reflections from incident electromagnetic radiation. Such surfaces are often employed in signature management applications to reduce radar returns. These applications are typically employed within the radio frequency portion of the electromagnetic spectrum.
As modern radar systems are often equipped with different and even multiple frequency bands, such signature management surfaces are preferably broad band, reducing reflections over a broad portion of the spectrum. Examples of known frequency selective surfaces providing such a response include one or more than one dielectric layers, which may be disposed above a ground plane. Thickness of the dielectric layers combined with the selected material properties reduce reflected radiation. The thickness of one or more of the layers is a predominant design criteria and is often on the order of one quarter wavelength. Unfortunately, such structures can be complicated and relatively thick, depending upon the selected dielectric materials and wavelength of operation, particularly since multiple layers are often employed.
The shapes can be selected to provide a resonant response having a preferred polarization. For example, surface features having an elongated shape provide a resonant response that is more pronounced in a polarization that is related to the orientation of the elongated shape. Thus, an array of vertically aligned narrow rectangles produces a response having a vertically aligned linear polarization. In general, preferred polarizations can be linear, elliptical, and circular.
The use of multiple frequency selective surfaces disposed above a ground plane, for radio frequency applications, is described in U.S. Pat. No. 6,538,596 to Gilbert. The frequency selective surfaces can include conductive materials in a geometric pattern with a spacing of the multiple frequency selective surface layers, which can be closer than a quarter wave. However, Gilbert seems to rely on the multiple frequency selective surfaces providing a virtual continuous quarter wavelength effect. Such a quarter wavelength effect results in a canceling of the fields at the surface of the structure. Thus, although individual layers may be spaced at less than one-quarter wavelength (e.g., λ/12 or λ/16), Gilbert relies on macroscopic (far field) superposition of resonances from three of four sheets, such that the resulting structure thickness will be on the order of one-quarter wavelength.